1. Field of Invention
This invention relates to a system for converting solar energy to electrical energy by an array of photovoltaic cells, specifically to a circuit for improving the conversion efficiency of such a system.
A number of systems have been developed to couple an electrical power source to a load; some of these are shown in the following U.S. patents: Hartman (U.S. Pat. No. 3,384,806, 1968), Ulle (U.S. Pat. No. 3,696,286, 1972), Steigerwald (U.S. Pat. No. 4,346,431, 1982), Norris (U.S. Pat. No. 4,468,569, 1984), Palm et al. (U.S. Pat. No. 4,530,043, 1985), Brasseur (U.S. Pat. No. 4,556,838, 1985), Bailey et al. (U.S. Pat. No. 4,580,090, 1986), Whittaker (U.S. Pat. No. 4,661,758, 1987), Mieth et al. (U.S. Pat. No. 4,695,785, 1987), Sakurai et al. (U.S. Pat. No. 4,816,739, 1989), and my own patent (Lafferty, U.S. Pat. No. 4,873,480, 1989).
The systems of Anthony, Palm, Whittaker, Bobier, and Sakurai are not directly concerned with the efficiency of power transfer to a load. While the coupling networks of Hartman, Ulle, Norris, Brasseur, Bailey, Mieth, and Lafferty employ some form of switching regulator to ensure efficient power transfer from source to load, only hartman, Ulle, Norris, Bailey, Mieth, and Lafferty teach systems for improving the efficiency of the conversion of power from photovoltaic (PV) sources to a load. The Lafferty system has the further advantage of providing temperature compensation.
In these systems, the current flow from the source is controlled by a regulator to keep the load impedance adjusted to equal the source impedance. Under this condition, maximum power transfer is obtained. However, the output voltage is not regulated, i.e., it varies as a function of the input voltage and the load current. Thus, many of these converters do not supply a usable output voltage because most applications of PV sources need a regulated output voltage. Also, the efficiency of the conversion process suffers when output voltage variations exceed usable limits.
In other words, none of the prior-art converters uses a regulator circuit that is capable of maintaining a fixed output voltage while delivering maximum available power to the load. In each case, the output voltage is a function of the load current, as well as the input voltage. Simply put, the unregulated output voltage sharply limits the utility and efficiency of PV sources.